1. Field of the Invention
This invention relates to lasers and, more particularly, to lasers which operate at a predetermined and fixed wavelength.
2. Background Art
Although lasers are usually thought of as sources of monochromatic light, all lasers actually emit light at multiple wavelengths simultaneously. For example, a CO.sub.2 laser can be made to emit at least 400 individual wavelengths in the 8.7-11.8 micron range when isotopes in addition to .sup.12 C.sup.16 O.sub.2 are used. Moreover, in most lasers, the width of the broadened laser line contains several longitudinal cavity modes which permits the laser to oscillate at several closely-spaced wavelengths at once.
Many laser applications require that the laser output be at a particular and constant wavelength. For example, applications in holography often require long coherence lengths that can be achieved only if the laser operates in a single, longitudinal mode. Applications in spectroscopy, photochemistry and isotope separation require a laser having a specific wavelength and a narrow bandwidth.
A wide variety of arrangements are known for controlling a laser to operate at a particular wavelength. As a general rule, it is desirable to selectively increase the cavity loss for the other wavelengths where the active medium produces gain, but without increasing loss at the desired wavelength. Known arrangements include such passive devices as tuning prisms, diffraction gratings, filters positioned at Brewster's angle, Fabry-Perot etalon plates and tuning wedges. However, these devices have a number of disadvantages. For example, prisms are not particularly effective in selecting one wavelength from a plurality of closely-spaced wavelengths. Diffraction gratings are difficult and expensive to manufacture and also introduce substantial energy losses at the desired wavelength. As a result, the laser must be operated at higher input power levels to obtain the same output power level, thus adding expense and inefficiencies to the laser system.
The operating wavelength of a laser can also change due to physical changes in the cavity length as the system operates, generally changes from expansion or contraction due to temperature variations. The highly reflective mirror at one end of the laser cavity has been mounted on a piezoelectric crystal to change the position of the mirror as the cavity length changes. The problem with such an arrangement is that the laser system must additionally include complicated and expensive devices for measuring deviations from the desired wavelength and energizing the piezoelectric crystal to compensate for any changes in wavelength.
Mirrors which reflect only at the desired wavelength and at no other wavelength have also been proposed. Other arrangements for controlling the operating wavelength of a laser are shown in U.S. Pat. Nos. 4,305,046; 4,558,452; 4,615,034; and 4,701,924.
It is an object of the present invention to provide a passive arrangement for accurately, reliably and inexpensively selecting a particular operating wavelength for a laser, but without diminishing the output of the laser.